The modulation of optical signals at microwave and millimeter-wave frequencies is of much interest for fiber-optic communications and other optical signal processing. The electrooptic effect, which induces an electric-field dependent change in refractive index, is known to operate at frequencies well in excess of 100 gigahertz. The change in refractive index can induce phase shifts in optical signals, and in appropriate configurations can also modulate the intensity, frequency, or polarization state of optical signals. However, it is hard to achieve the amount of overlap of the microwave signal and the optical signal that is required for efficient electrooptic interaction.
The usual technique for achieving a strong electrooptic overlap is to apply the microwave signal to metallic conductors in proximity to an optical waveguide fabricated in an electrooptic material, such as lithium niobate. The microwave signal induces an electric field within the optical waveguide causing a change in the refractive index, which changes the optical wave propagation.
It is difficult to achieve the proper microwave impedance with the metallic conductors. Moreover, the DC resistance associated with the thin metal structure causes severe restrictions on the high frequency operation. There is also a large velocity mismatch between the electrical signal propagating on the metallic electrode and the optical signal propagating in the optical waveguide. This mismatch limits the interaction length over which the optical signal accumulates the electrooptically induced phase shift.
To overcome the velocity mismatch several methods of "artificial velocity matching" have been proposed such as "phase reversed electrodes" and "intermittent interaction electrodes". These methods, which rely on special design of the metallic stripline, present increasing fabrication problems as the wavelength of the electrical signal is reduced for operation at millimeter wavelengths..sup.1
The above limitations are absent in the present invention, which does not require metallic striplines. The high-frequency electrical signal is propagated in a dielectric waveguide collinearly with the optical signal at substantially matching phase velocities..sup.2